Spark plug

ABSTRACT

A spark plug comprising a center electrode, a first noble metal tip joined to the center electrode, an insulator, a metallic shell, a ground electrode joined to the metallic shell and including an outer layer and an inner layer, and a second noble metal tip joined to the ground electrode by way of a melted portion, wherein in a cross section of the ground electrode, the protrusion height A of the second noble metal tip is 0.4 mm or more, the ground electrode includes a substantially flat joining surface to which the second noble metal tip is joined and an outwardly curved surface, the inner layer has at the joining surface side a substantially flat surface or recessed surface, and the minimum distance F between the melted portion and the inner layer is 0.1 mm or more.

TECHNICAL FIELD

The present invention relates to a spark plug for use with an internalcombustion engine.

BACKGROUND TECHNIQUE

A spark plug for an internal combustion engine such as an automotiveengine is provided with, for example, a center electrode, an insulatorprovided at the outside of the center electrode, a tubular metallicshell provided at the outside of the insulator and a ground electrodewith a base end portion joined to a front end surface of theabove-described metallic shell. The ground electrode has a substantiallyrectangular cross section and is disposed so that its front end portioninner surface faces a front end portion of the above-described centerelectrode, whereby a spark discharge gap is formed between the front endportion of the center electrode and the front end portion of the groundelectrode.

The metallic shell is formed at an outer circumferential surface with anunshown thread portion. The spark plug is threadedly attached to a sparkplug hole having a female thread and formed in a cylinder head of anengine. In the meantime, in case in an installed state of the sparkplug, the mixture gas has such a positional relationship of lashingagainst a back face of the ground electrode, there is a fear of theground electrode obstructing flow of a mixture gas into a sparkdischarge gap. As a result, there is a fear of variations in theignitability being caused.

In contrast to this, there is a technique that in a spark plug of thetype having two or more ground electrodes, each ground electrode isformed into a cylindrical shape having a substantially circular crosssection (refer to, for example, Patent Document 1). By making, in thismanner, the cross section be substantially circular-shaped, the mixturegas is hard to come off from the ground electrode but is caused to turnaround the ground electrode into the inside thereof even when themixture gas has such a positional relationship of lashing against theback face of the ground electrode, thus enabling the mixture gas toreach the spark discharge gap easily.

Further, there is a technique that the cross section of the groundelectrode is formed into a substantially trapezoidal shape (refer to,for example, Patent Document 2). By making, in this manner, the crosssection be substantially trapezoidal-shaped, it can be said that themixture gas reaches the spark discharge gap more easily as compared withthe case the cross section is rectangular.

Patent Document 1: Unexamined Japanese Patent Application PublicationNo. 11-121142

Patent Document 2: Unexamined Japanese Patent Application PublicationNo. 5-13146

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, for the reason that the ground electrode is to be joined to thefront end surface of the metallic shell, if the cross section of theground electrode is circular-shaped, its sectional area inevitablybecomes smaller as compared with the case where the cross section isrectangular-shaped. As a result, there is a fear of a so-calledheat-drawing (heat radiation ability) becoming worse, and the electrodetemperature rising easily at high-speed driving and the like such thatthe rate of exhaustion of the ground electrode becomes larger and itsdurability is lowered.

Further, while in recent years, it has also been considered to join tipsmade of noble metal alloy (noble metal tips) to a front end portion of acenter electrode and a front end portion of a ground electrode,respectively and thereby improve the ignitability and the sparkpropagation ability, there is an anxiety that in case the cross sectionof the ground electrode is circular-shaped as the above-described PatentDocument 1, it is worried that soldering of the noble metal tip on theground electrode side might become difficult.

Further, while in Patent Document 2, it is described that the groundelectrode includes an outer layer and an inner layer that is superior inthermal conductivity to the outer layer and there is a suggestion aboutimprovement in the heat-drawing ability, there is not any descriptionabout a noble metal tip joining method. In this connection, if, forexample, the noble metal tip is welded by resistance welding, asufficient joining strength cannot be obtained. Further, in case, forexample, the noble metal tip is welded by laser or electron beam weldingor the like, there is a fear that a welded portion might be extendedover the inner layer, and in such a case, there is a fear that theoxidation-resistant property is lowered due to formation of oxidationscales.

The present invention has been made in view of the above-describedcircumstances and has for its object to provide a spark plug for aninternal combustion engine, which can inhibit obstruction to flow of amixture gas into a spark discharge gap, prevent lowering of theignitability and improve the joining strength of a noble metal tipthereby improving the durability.

Means for Solving the Problem

Hereinafter, each structure suited to solve the above-described problemand the like will be described by being itemized. In the meantime,according to the necessity, an operation effect peculiar to acorresponding structure will be described additionally.

A spark plug according to a first structure of the present inventionincludes a rod-shaped center electrode, a first noble metal tip joinedto a front end of the center electrode, a substantially cylindricalinsulator provided at an outer circumference of the center electrode, atubular metallic shell provided at an outer circumference of theinsulator, a ground electrode having a base end portion joined to afront end surface of the metallic shell, and a front end portion facinga front end portion of the center electrode, the ground electrodeincluding an outer layer made of nickel alloy and an inner layer made ofa material having a better thermal conductivity than the outer layer,and a second noble metal tip joined to the front end portion of theground electrode by way of a melted portion formed by laser welding orbeam welding and forming a spark discharge gap between the second noblemetal tip and the first noble metal tip, wherein in a cross section ofthe ground electrode as viewed from the front end surface side of theground electrode along the axis of the second noble metal tip, thefollowing features (1) to (4) are provided. Namely, (1) the protrusionheight A of the second noble metal tip from the joining surface to thefront end of the second noble metal tip is 4 mm or more, (2) the groundelectrode has a substantially flat joining surface to which the secondnoble metal tip is joined and an outwardly curved surface, (3) the innerlayer has at the joining surface side a substantially flat surface orrecessed surface, and (4) the minimum distance F between the meltedportion and the inner layer is 0.1 mm or more.

By the above-described first structure, the ground electrode has anoutwardly curved surface. For this sake, the mixture gas turn aroundalong the curved surface and into the inside of the ground electrode andreaches the spark discharge gap with ease. As a result, it becomespossible to prevent the ignitability from being lowered.

Further, the ground electrode has at least a spark discharging portionan outer layer made of nickel alloy and an inner layer made of metal ofa better heat conductivity than the outer layer. The existence of theouter layer can elevate the durability against oxidation, while at thesame time the existence of the inner layer can improve the heat drawing,whereby a drawback due to a rise of a ground electrode temperature athigh-speed driving and the like, such as increase of a spark dischargegap due to consumption of the ground electrode, can be inhibited withease.

Further, to the center electrode and the ground electrode are weldedrespective noble metal tips to improve the spark consumption resistanceunder high temperature. Particularly, with the first structure, theprotrusion height A from the joining surface of the ground electrode tothe front end of the second noble metal tip is set to 0.4 mm or moresuch that the more improvement in the ignitability can be attained.

Further, with the first structure, at least the joining surface of theground electrode, to which the second noble metal tip is joined, issubstantially flat-shaped. For this reason, as compared with the casethe joining surface is formed into a curved surface, complication of thejoining work can be avoided with ease and improvement in the joiningstrength can be attained.

Furthermore, the second noble metal tip is welded to the joining surfaceby way of a melted portion formed by the metal constituting the secondnoble metal tip and the metal constituting the outer layer of the groundelectrode, which are melted and mixed with each other by being subjectedto at least laser beam welding or electron beam welding. For thisreason, the joining strength of the second noble metal tip can beimproved and the joining state can be stabilized further.

Furthermore, with this first structure, the joining surface side (secondnoble metal tip side) of the second noble metal tip is substantiallyflat-shaped or recess-shaped. For this sake, even if the depth of themelted portion is set relatively large, the shortest distance F betweenthe melted portion and the inner layer can be attained by 0.1 mm or morewith ease. Accordingly, improving the joining strength of the secondnoble metal tip while inhibiting decrease in the resistance tooxidation, which are, so to speak, conflicting effects, can be attainedat a stroke.

In the meantime, as a method of joining the second noble metal tip tothe ground electrode can be cited welding such as laser welding orelectron beam welding that can form a melted portion as described above.However, it is more desirable to perform temporary attachment byresistance welding, prior to the above-described laser welding orelectron beam welding, than performing such welding without anypretreatment.

Further, even if the melted portion formed by laser welding or electronbeam welding is relatively small so that there exists between the secondnoble metal tip and the outer layer an area in which there is not anymelted portion, joining between the second noble metal tip and the outerlayer can be obtained securely since resistance welding is performedbeforehand, thus wiping away a fear of falling off of the second noblemetal tip and the like.

Further, while it has been described as above that the outer layer ofthe ground electrode is made of nickel alloy, it is desirable that atleast a portion of the inner layer that is made of a material having abetter thermal conductivity than the outer layer is made of a materialincluding copper as a major constituent. By having the inner layer whosemajor constituent is copper, good heat drawing can be attained, and adrawback that is caused by temperature rise of the ground electrode andthe second noble metal tip can be inhibited more assuredly. In themeantime, the ground electrode is not limited to a two-layer structurebut may be of a structure of three layers or more. However, the innerlayer needs to contain a metal having a better thermal conductivity thanthe outer layer. Accordingly, in case, for example, there are inside theouter layer an intermediate layer made of a copper alloy or pure copperand inside the intermediate layer an innermost layer made of purenickel, the inner layer can be construed as being constituted by theintermediate layer and the innermost layer.

Further, for obtaining a sufficient joining strength, it is desirablethat in the above-described cross section, the depth E of the meltedportion from the joining surface toward the inner layer along the axialdirection is 0.1 mm or more.

Further, in consideration of the aspect of production, it is desirableto employ the following structures 6 to 9.

Further, a spark plug according to a second structure of the presentinvention includes a rod-shaped center electrode, a first noble metaltip joined to a front end of the center electrode, a substantiallycylindrical insulator provided at an outer circumference of the centerelectrode, a tubular metallic shell provided at an outer circumferenceof the insulator, a ground electrode having a base end portion joined toa front end surface of the metallic shell, and a front end portionfacing a front end portion of the center electrode, the ground electrodeincluding an outer layer made of nickel alloy and an inner layer made ofa material having a better thermal conductivity than the outer layer, asecond noble metal tip joined to the front end portion of the groundelectrode by way of a melted portion formed by laser welding or beamwelding and forming a spark discharge gap between the second noble metaltip and the first noble metal tip, wherein in a cross section of theground electrode as viewed from the front end surface side of the groundelectrode along the axis of the second noble metal tip, the followingfeatures (1) to (4) are provided. Namely, (1) the protrusion height A ofthe second noble metal tip from the joining surface to the front end ofthe second noble metal tip is 0.4 mm or more, (2) the ground electrodeincludes a substantially flat joining surface to which the second noblemetal tip is joined and an outwardly curved surface, (3) the inner layerhas at the joining surface side a substantially flat surface or recessedsurface, and (4) the melted portion is disposed at a distance from thejoining surface.

In such a second structure, the features (1) to (3) are common with thefirst structure but the feature (4) differs.

Namely, in the second structure, since the second noble metal tip iswelded to the joining surface of the ground electrode by way of theintermediate member, the melted portion is formed between the secondnoble metal tip and the intermediate member and is positioned at adistance from the joining surface. Accordingly, there is not any fear ofthe welded portion reaching the inner layer to lower the resistance tooxidation. In the meantime, it is desirable to make the intermediatemember by the same nickel alloy as the ground electrode and join them byresistance welding.

Further, in the second structure, by using the intermediate member, theprotrusion height A of the second noble metal tip from the joiningsurface to the front end of the second noble metal is set to 4 mm ormore. Namely, since a portion of the protrusion height A can beconstituted by an intermediate member, the amount of noble metal usedcan be reduced.

Further, for securing the heat radiation ability of the noble metal tipwhile maintaining the effect of reducing the amount of noble metal used,it is preferable to set, in the above-described cross section, theshortest distance T between the joining surface and the inner layersmaller than the protrusion height H of the intermediate member from thejoining surface.

Further, for performing, in the spark plugs of the first and secondstructures, heat radiation of the inner layer effectively, it ispreferable that the shortest distance T between the joining surface andthe inner layer is 0.4 mm or less.

Further, for performing heat radiation by the inner layer effectively,it is preferable that the inner layer of a sufficient width ispositioned just under the second noble metal tip. Specifically, assumingW denotes the width of the front end surface of the second noble metaltip in the above-described cross section and C denotes the width of theinner layer in the direction parallel to the joining surface in theabove-described cross section, it is preferable to satisfy W≦C

In addition, in the spark plugs of the first and second structures, itis preferable to form the ground electrode including the joining surfaceby swaging.

In general, as a method of forming a metallic material so that thematerial becomes slender and round are cited, for example, a drawingprocess using a dice or the like, an extrusion process using a femaledie or the like, a cutting process, electro discharge machining, or thelike. However, from a point of view of stably producing a groundelectrode that has a plural-layer structure and is relatively thin andsubstantially circular in section, it is difficult to employ any of theabove-described machining and processes independently. For example, byonly the drawing process or the protrusion process, it is actuallydifficult to make smaller the diameter so that the diameter is equal toor less than 1.5 mm and the cost is increased. Further, by the cuttingprocess and the electro machining process, a variation of each productis liable to be caused, and further a variation in the central positionof the inner layer relative to the ground electrode is liable to becaused. Furthermore, the machining work is inefficient, thus increasingthe cost.

In contrast to this, by performing swaging, the ground electrode can beobtained stably and without difficulty and the spark plug can beproduced.

Further, by performing swaging, the machining rate of the portion wherethe joining surface (center electrode side) is provided becomes larger,and the hardness can be made larger.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway elevational view showing a structure of aspark plug according to a first embodiment.

FIG. 2 is an enlarged fragmentary sectional view of the spark plug.

FIG. 3 is a side view showing a spark plug as viewed in the direction atright angles to FIG. 2.

FIG. 4 is a plan view showing a state of the spark plug as viewed from afront end side.

FIG. 5 is a schematic sectional view of a ground electrode, etc. asviewed from a front end side of the ground electrode along an axis.

FIG. 6A is a schematic sectional view showing a production process ofthe ground electrode.

FIG. 6B is a schematic sectional view showing a production process ofthe ground electrode.

FIG. 6C is a schematic sectional view showing a production process ofthe ground electrode.

FIG. 7A is a schematic sectional view showing a production process ofthe ground electrode.

FIG. 7B is a schematic sectional view showing a production process ofthe ground electrode.

FIG. 7C is a schematic sectional view showing a production process ofthe ground electrode.

FIG. 8 is a schematic sectional view of a ground electrode, etc. in asecond embodiment as viewed from a front end side of the groundelectrode along an axis.

FIG. 9A is a schematic sectional view showing a production process ofthe ground electrode in the second embodiment.

FIG. 9B is a schematic sectional view showing a production process ofthe ground electrode in the second embodiment.

FIG. 9C is a schematic sectional view showing a production process ofthe ground electrode in the second embodiment.

FIG. 10A is a schematic sectional view showing a production process ofthe ground electrode in the second embodiment.

FIG. 10B is a schematic sectional view showing a production process ofthe ground electrode in the second embodiment.

FIG. 10C is a schematic sectional view showing a production process ofthe ground electrode in the second embodiment.

FIG. 11 is a schematic sectional view for explanation of the concept ofthe oxidation scale ratio.

FIG. 12 is a schematic sectional view of a ground electrode, etc. in athird embodiment as viewed from a front end side of the ground electrodealong an axis.

FIG. 13 is a schematic sectional view of a ground electrode, etc. in afourth embodiment as viewed from a front end side of the groundelectrode along an axis.

FIG. 14 is a schematic sectional view of a ground electrode etc. in afifth embodiment as viewed from a front end side of the ground electrodealong an axis.

FIG. 15 is a perspective view schematically showing a ground electrode,etc. in a further embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Hereinafter, the first embodiment will be described with reference todrawings. FIG. 1 is a partially cutaway elevational view showing a sparkplug 1. In the meantime, description will be made by regarding, in FIG.1, an axial CL1 direction of the spark plug 1 as a vertical direction inthe drawing, the lower side as the front end side of the spark plug 1and the upper side as the rear end side.

The spark plug 1 is constituted by an insulator 2 having a long shape, atubular metallic shell 3 holding the insulator and so on.

The insulator 2 is formed with an axial hole 4 extending therethroughalong an axis CL1. Into the front end side of the axial hole 4 isinserted and fixed thereat a center electrode 5. Further, into the rearend side of the axial hole 4 is inserted and fixed thereat a terminalelectrode 6. Between the center electrode 5 and the terminal electrode 6within the axial hole 4 is disposed a resistor 7, and the resistor 7 iselectrically connected to the center electrode 5 and the terminalelectrode 6 by way of respective conductive glass seal layers 8 and 9.

To a front end of the center electrode 5 protruding from the front endof the insulator 2 is welded a first noble metal tip 31 containing 5 wt.% of platinum.

On the other hand, the insulator 2 is formed by sintering alumina, etc.as is well known and has at an outer periphery a large diameter portion11 positioned substantially in the middle part in the axial CL1direction and in the form of a flange protruding radially outward, amiddle portion 12 positioned more on the front end side than the largediameter portion 11 and formed so as to be smaller in diameter than thelarge diameter portion, and a leg portion 13 position more on the frontend side than the middle portion 12, formed so as to be smaller indiameter than the middle portion and adapted to be exposed to acombustion chamber of an internal combustion engine (engine). The frontend side of the insulator 2, including the large diameter portion 11,the middle portion 12 and the leg portion 13 is accommodated in thetubular metallic shell 3. At a connecting part between the leg portion13 and the middle portion 12 is provided a shoulder portion 14, and theinsulator 2 is lockingly engaged at the shoulder portion 14 with themetallic shell 3.

The metallic shell 3 is formed by low-carbon steel or the like metalinto a tubular shape and formed at the outer circumferential surfacewith a thread portion (male thread portion) 15 for attaching the sparkplug 1 to the cylinder head of the engine. The outer circumferentialsurface on the rear end side of the threaded portion 15 is formed with aseat portion 16, and a gasket 18 is fitted on a neck portion 17 at therear end of the threaded portion 15. Further, on the rear end side ofthe metallic shell 3 is provided a tool engagement portion 19 having ahexagonal cross section and engaged by a wrench or the like tool at thetime of attachment of the metallic shell 3 to the cylinder head, and atthe same time at the rear end portion is provided a crimped portion 20for holding the insulator 2.

Further, at the inner circumferential surface of the insulator 2 isprovided a shoulder portion 21 for locking engagement with the insulator2. The insulator 2 is inserted from the rear end side of the metallicshell 3 toward the front end side, and is fixed under a condition of itsshoulder 14 being lockingly engaged with the shoulder portion 21 of themetallic shell 3 by crimping a rear end side opening portion of themetallic shell 3 radially inward, i.e., by forming the above-describedcrimped portion 20. In the meantime, between the shoulder portions 14and 21 of both of the insulator 2 and the metallic shell 3 is interposeda circular ring-shaped plate packing 22. By this, the airtightness ofthe combustion chamber is maintained, and the fuel-air coming into thespace between the leg portion 13 of the insulator 2 and the innercircumferential surface of the metallic shell 3, which are exposed tothe inside of the combustion chamber, is prevented from leakingtherefrom.

Further, for making the airtight by the crimping more assured, circularring members 23 and 24 are provided on the rear end side of the metallicshell 3 and interposed between the metallic shell 3 and the insulator 2,and power of talc (talc) 25 is filled up between the ring members 23 and24. Namely, the metallic shell 3 holds the insulator 2 by way of theplate packing 22, ring members 23, 24 and talc 25.

Further, to the front end surface of the metallic shell 26 is joined asubstantially L-shaped ground electrode 27. Namely, the ground electrode27 is welded at the base end portion to the front end surface 26 of themetallic shell 3 while being bent at the front end side, and is disposedso that one side surface on the front end side thereof faces the firstnoble metal tip 31. To the front end portion of the ground electrode 27is provided a second noble metal tip 32 in such a manner as to face thefirst noble metal tip 31. A gap between the first and second noble metaltips 31 and 32 is adapted to serve as a spark discharge gap 33. In themeantime, the axes of the first and second noble metal tips 31 and 32are disposed so as to coincide with the axis CL1 so that the axis CL1serves as both of the axes of the first and second noble metal tips 31and 32.

As shown in FIG. 2, the center electrode 5 is constituted by an innerlayer 5A made of copper or copper alloy and an outer layer 5B made ofnickel (Ni) alloy. The center electrode 5 is decreased in diameter onthe front end side and generally rod-shaped (cylindrical-shaped), andits front end surface is formed so as to be flat. On the front endsurface is laid the above-described first noble metal tip 31, and alongan outer peripheral portion of that joining surface is performed laserwelding or electro beam welding thereby allowing the first noble metaltip 31 and the center electrode 5 to melt together and form a meltedportion 41. Namely, the first noble met al tip 31 is joined to the frontend of the center electrode 5 by sticking thereto at the melted portion41.

On the other hand, the ground electrode 27 is of a two-layer structureincluding an inner layer 27A and an outer layer 27B. The outer layer 27Bof this embodiment is formed by nickel alloy such as Inconel 600 orInconel 601 (either is registered trademark). In contrast to this, theinner layer 27A is formed by copper alloy or pure copper that is a metalhaving a better thermal conductivity than the above-described nickelalloy. By the existence of the inner layer 27A, improvement in the heatdrawing ability is attained (detailed description in this connectionwill be made hereinlater). In the meantime, in this embodiment, theground electrode is described as being of a simple two-layer structurefor convenience of description but it may be of a three-layer structureor of a multilayer structure with layers equal to or more than fourlayers. However, as against the outer layer 27B, the layer inside theouter layer needs to contain a metal having a better thermalconductivity than the outer layer 27B. Accordingly, for example, anintermediate layer made of a copper alloy or pure copper may be providedinside the outer layer 27B, and an innermost layer made of pure nickelmay be provided inside the intermediate layer. In this case, the innerlayer 27A is constituted by the intermediate layer and the innermostlayer.

Further, in this embodiment, the ground electrode 27 is shaped to havesuch a circular cross section that is partly crushed. Of the groundelectrode 27, a front end including at least a portion to which thesecond noble metal tip 32 is joined (in this embodiment, all the area inthe longitudinal direction), is swaged so as to have a substantiallyflat surface shape. Although the method of this processing will bedescribed in detail later, by the above-described swaging, the centerelectrode 5 side surface of the outer layer 27B is formed with a flatsurface F1. In other words, the outer peripheral shape of the crosssection of the ground electrode 27 as viewed from the front end surfaceof the ground electrode 27 along the axis CL1 is of such a shape that isobtained by cutting off a segment from a substantially circular shape sothat a more than half part of the circular shape remains. In thisembodiment, by going through the swaging, the center electrode 5 sideportion of the outer electrode 27B has a larger hardness than the backsurface side portion that is on the side opposite to the centerelectrode 5.

Further, while it has already been described that the first noble metaltip 31 on the center electrode 5 side contains iridium as a majorconstituent, the second noble metal tip 32 on the ground electrode 27side is made of a noble metal alloy containing, for example, platinum asa major constituent and 20% rhodium. However, such a material structureis only for the purpose of illustration, the above description is notfor the purpose of limitation. These first and second noble metal tips31 and 32 are produced, for example, as follows. First, an ingotcontaining iridium or platinum as a major constituent is prepared, eachalloy constituent is mixed and melted so that the above-describedpredetermined composition is obtained, from this melted alloy an ingotis formed again, and thereafter the ingot is processed by hot-forgingand hot rolling (groove rolling). Then, after a rod-shaped material isobtained by a drawing process, it is cut to a predetermined lengththereby obtaining the first and second noble metal tips 31 and 32 in theform of a cylinder.

By the way, as shown in FIG. 5, the second noble metal tip 32 on theground electrode 27 side in this embodiment is directly joined to thefront end portion (flat surface F1) of the ground electrode 27. Morespecifically, the second noble metal tip 32 is first temporarilyattached to the flat surface F1 by resistant welding. In addition tothat, along the outer peripheral portions of the abutting surfaces isperformed laser welding or electron beam welding. By this, the secondnoble metal tip 32 and the outer layer 27B are melted together to formthe melted portion 42 thereby making the second noble metal tip 32 andthe ground electrode 27 be firmly joined and fixed. However, the meltedportion 42 is not extended to the inner layer 27A, i.e., the meltedportion 42 is in a non-contact state with the inner layer 27A.

Further, in this embodiment, the protrusion height A from the joiningsurface of the second noble metal tip 32, i.e., the flat surface F1, tothe front end of the second noble metal tip 32 is set to 0.4 mm or more.Further, in a cross section of the ground electrode 27 as viewed fromthe front end side of the ground electrode 27 along the axis CL1 of thenoble metal tip 32, the second noble metal tip 32 side of the innerlayer 27A is substantially flat-shaped.

Further, the depth E of the melted portion 42 from the flat surface(joining surface) F1 toward the inner layer 27A along the axial CL1direction of the above-described melted portion 42 is set to 0.1 mm ormore, and the shortest distance between the melted portion 42 and theinner layer 27A is set to 0.1 mm or more. Further, the shortest distancebetween the flat surface (joining surface) F1 and the inner layer 27A isset to 0.4 mm or less. Further, the outer diameter W of the second noblemetal tip 32 and the lateral width C of the inner layer 27A satisfy W-C.

Then, the production method of the spark plug 1 structured as describedabove will be described by attaching importance to the production methodof the above-described ground electrode 27. First, the metallic shell 3is prepared beforehand. Namely, a metallic material (for example, aferrous material such as S15C or S25C, or stainless steel material)formed into a cylindrical shape is formed with a through hole by coldforging thereby formed into a rough shape. Thereafter, by performingcutting, the outer shape is fixed thereby obtaining a metallic shellintermediate article.

On the other hand, an intermediate article of the ground electrode 27 isproduced. Namely, the intermediate article of the ground electrode 27 isin the form of a straight rod before being bent. The ground electrode 27before being bent is, for example, obtained as follows.

Namely, as shown in FIG. 6A, a core material 51 made of a metallicmaterial for forming the inner layer 27A and a bottomed tubular body 52made of a metallic material for forming the outer layer 27B areprepared. The core material 51 includes a cylindrical pedestal portion53 and a cylindrical core portion 54 protruding upward from the centerof the upper surface of the pedestal portion 53 and integrally formedtherewith. The cross sectional area of the core portion 54 is set largerthan that of the inner layer 27A. On the other hand, the bottomedtubular body 52 has a recessed portion 55 of an equal size to the coreportion 54 and a bottomed portion 56 as is so named. Further, the outerperipheral wall of the recessed portion 55 is set thicker than the outerlayer 27B. As shown in FIG. 6B, the core portion 54 of the core material51 is inserted into the recessed portion 55 of the bottomed tubular body52 thereby forming a cup material 57 having a core-sheath structure.Then, the cup material 57 is subjected to a cold thinning processthereby forming a rod-shaped body 271 as shown in FIG. 6C. As the coldthinning process in this embodiment is cited, for example, a wiredrawing process using a die, etc., an extrusion process using a femaledie, etc. and the like. In the meantime, the rod-shaped body 271 may besuch one that is cut along a plane J-J of FIG. 6C and removed therefroma portion corresponding to the above-described pedestal portion 53. Bysuch cutting and removing, the inner layer 27A is not exposed when theground electrode 27 is finally formed. Further, the external shape ofthe rod-shaped body 271 at this point in time may be optional and isformed into a cylindrical shape having a circular cross section in thisembodiment.

Then, to the front end surface of the above-described metallic shellintermediate article is joined the rod-shaped body 271 by resistancewelding. In the meantime, at the time of the resistance welding, thereis caused a so-called “expulsion” and a work for removing the“expulsion” is carried out.

Thereafter, the rod-shaped body 271 is processed by swaging. In thisconnection, to the front end surface of the metallic shell intermediatearticle has already been welded the rod-shaped body 271. For thisreason, at the time of swaging, under a condition where the metallicshell intermediate article is held, the rod-shaped body 271 welded tothe metallic shell intermediate article can be introduced from the frontend side thereof to a working portion (swaging die) of a swagingmachine. Accordingly, it becomes unnecessary to take the trouble to makethe ground electrode intermediate article longer for securing a holdingportion at the time of swaging and cut off the above-described holdingportion after swaging. As for a swager, it is desirable to use aplurality of swagers such as one only for making smaller in diameter andone for making smaller in diameter while forming a sectional shapehaving a flat surface F1 as in this embodiment, i.e., a so-calledcrushed shape. By the first step swaging, as shown in FIG. 7A, therod-shaped article 271 is made further reduced in diameter and by thesecond step swaging, as shown in FIG. 7B, further reduced in diameterwhile being formed with the flat surface F1 and forming a groundelectrode intermediate article 272 in which a part of the inner layer27A (the side to which the second noble metal tip 32 is welded later) isdeformed into a substantially flat shape. In the meantime, the groundelectrode intermediate article 272 may be welded to the front endsurface of the metallic shell intermediate article after the swagingprocess of the rod-shaped article 271.

After the above-described swaging process, a thread portion 15 may beformed by at a predetermined portion of the metallic shell intermediatearticle by rolling. By this, the metallic shell 3 to which the groundelectrode intermediate article 272 that is reduced in diameter butbefore being bent is welded is obtained. To the metallic shell 3 towhich the ground electrode intermediate article 272 is welded is appliedzinc plating or nickel plating. In the meantime, in order to improve thecorrosion resistance, the surface of the metallic shell may be processedby chromate treatment.

Further, as shown in FIG. 7C, to the front end portion of the groundelectrode intermediate article 272 is temporarily welded by resistancewelding as described above and, in addition to that, joined by laserwelding or electron beam welding the above-described noble metal tip 32.In the meantime, to make the welding more assured, removal of plating ofthe welding portion is performed prior to the welding or masking isprovided to the portion to be welded at the time of the plating process.Further, the welding of the tip may be performed after the assembly thatwill be described later.

On the other hand, the forming process of the insulator 2 is performedindependently of the metallic shell 3. For example, a raw grain materialfor forming the insulator is prepared by using a powder material mainlyconsisting of alumina and containing binder, etc., and by using this rawgrain material a tubular body is formed by rubber pressing. The tubularbody obtained is processed by grinding and fixed in shape. The bodyfixed in shape is inserted into a sintering furnace and sintered,whereby the insulator 2 is obtained.

Further, independently of the metallic shell 3 and insulator 2 isprepared the center electrode 5. Namely, a Ni-system alloy is processedby forging, and at its central portion is provided a copper core forimproving the heat radiation ability. To the front end portion of thecenter electrode is joined the first noble metal tip 31 by laser weldingor the like.

Then, the center electrode 5 to which the first noble metal tip 31 isjoined, which is obtained as described above, and the terminal electrode6 are sealingly fixed to the axial hole 4 of the insulator 2 by a glassseal layer 8. As the glass seal layer 8 is generally used one that isobtained by mixing borosilicate glass and metal powder and adjusting thesame. Then, the center electrode 5 is first put into a state of beingheld inserted into the axial hole 4, and after the terminal electrode isput into a state of being held pushed from the rear after the adjustedseal material is poured into the axial hole 4 of the insulator 2,sintering is performed within the sintering furnace. In the meantime, atthis time, a glazing layer may, at the same time, be formed by sinteringon a surface of a rear end side body portion of the insulator 2 or theglazing layer may be formed beforehand.

Thereafter, the insulator 2 equipped with the center electrode 5 andterminal electrode 6 that are respectively prepared as described aboveand the metallic shell 3 equipped with the ground electrode intermediatearticle 272 to which the second noble metal tip 32 is welded areassembled together. More specifically, by cold crimping or hot crimpingthe rear end portion of the metallic shell 3, which is formed relativelythin, the insulator 2 is partially and circumferentially surroundedwithin the metallic shell 3 and held within the same.

Finally, by bending the ground electrode intermediate article 272,adjustment of the spark discharge gap 33 between the center electrode 5(the first noble metal tip 31 thereof) and the ground electrode 27 (thesecond noble metal tip 32 thereof) is executed.

By passing through such a series of processes, the spark plug 1 havingthe above-described structure is produced.

As having been described in detail, according to the present invention,with respect to the spark plug 1 obtained, the ground electrode 27 hasmore at the front end side than at least the center of the sparkdischarge gap 33 and at the back surface on the side opposite to thecenter electrode 5 an outwardly curved surface (circular arc-shaped incross section). For this reason, as shown, for example, in FIGS. 3 and4, even if the mixed gas has such a positional relation as to directlystrike the back surface of the ground electrode 27, it is easy for themixed gas to turn around into the inside of the ground electrode 27 andreach the spark discharge gap 33. As a result, it becomes possible toprevent the ignitability from being lowered. Further, since the frontend of the second noble metal tip 32 protrudes more toward the firstnoble metal tip 31 side than the imaginary circle 27C that is formed byextending the circular arc shape of the curved surface, the dischargevoltage can be lowered.

Further, the ground electrode 27 has the outer layer 27B made of nickelalloy or the like and the inner layer 27A made of metal having a betterthermal conductivity than the outer layer 27B. For this reason, theinner layer 27A works for active heat radiation such that the so-called“heat drawing” becomes better. Accordingly, at high-speed driving or thelike, it becomes possible to inhibit a drawback due to a temperaturerise of the ground electrode 27 and the second noble metal tip 32, i.e.,lowering of the durability such as the oxidation resistance and wearresistance.

Further, in this embodiment, the second noble metal tip 32 of the groundelectrode 27 is joined to the flat surface F1 substantially in the formof a plane. For this sake, as compared with the case where the joiningsurface is formed into a curved surface, complication of the joiningwork can be avoided with ease and improvement in the joining strengthcan be attained.

Moreover, the second noble metal tip 32 is joined to the flat surface(joining surface) F1 by way of the melted portion 42 that is formed byprocessing of laser welding or electron beam welding. For this sake,improvement of the joining strength of the second noble metal tip 32 isattained and more stabilization of the joining state is attained.

Furthermore, the melted portion 42 is in a non-contact state with theinner layer 27A. For this sake, it becomes possible to inhibit loweringof the oxidation resistance due to contact between the melted portion 42and the inner layer 27 a, which causes formation of oxidized layer. Onthe other hand, in order to improve the joining strength of the secondnoble metal tip 32, it is desired that the melted portion 42 is formeddeep. In this connection, in a cross section of the ground electrode 27as viewed from the front end surface side of the ground electrode 27along the axis CL1, the second noble metal tip 32 side shape of theinner layer 27A is substantially flat. For this sake, even when thedepth of the melted portion 42 is made relatively large, it is hard forthe melted portion 42 and the inner layer 27A to contact. Accordingly,it becomes possible to improve the joining strength of the second noblemetal tip 32 while inhibiting lowering of the oxidation resistance.

Second Embodiment

Then, the second embodiment will be described with reference to FIGS. 8to 10. However, in the second embodiment, the same reference charactersare used for the same or like parts as the first embodiment while theirduplicate description being omitted, and the different point from thefirst embodiment will be mainly described.

In the first embodiment, in the cross section of the ground electrode 27as viewed from the front end surface side of the ground electrode 27along the axis CL1, the second noble metal tip 32 side shape of theinner layer 27A is substantially flat. In contrast to this, thisembodiment features that, as shown in FIG. 8, the second noble metal tip32 side shape of the inner layer 27 is recessed.

The ground electrode 27 is obtained, for example, in the followingmanner. Namely, as shown in FIG. 9A, a core member 51 made of a metallicmaterial constituting the inner layer 27A and a bottomed tubular body 52made of a metallic material constituting the outer layer 27B are firstprepared. The core member 51 includes a cylindrical pedestal portion 53and a core portion 54 integrally formed so as to protrude upward fromthe upper surface center of the pedestal portion 53 and having acylinder partially and longitudinally cut off. On the other hand, thebottomed tubular body 52 has a recessed portion 55 and a bottom portion56 which are of the same size and shape as the above-described coreportion 54.

Then, as shown in FIG. 9B, by fitting the core portion 54 of the coremember 51 in the recessed portion 55 of the bottomed tubular body 52, acup member 57 of a core-sheath structure is formed, and by treating thecup member 57 by a cold thinning process, a rod-shaped body 271 as shownin FIG. 9C is formed. Of course, similarly to the first embodiment, onefrom which a portion corresponding to the pedestal portion 53 is cut offby a plane passing the line J2-J2 of FIG. 9C may be employed as therod-shaped body 271.

Then, the rod-shaped body 271 is joined to the front end surface of theof the above-described metallic shell intermediate article by resistantwelding, and similarly to the first embodiment, a swaging process of therod-shaped body 271 is performed. Namely, by the first step swaging asshown in FIG. 10A, the rod-shaped body 271 is further reduced indiameter, and by the second step swaging is formed, as shown in FIG. 10Ba ground electrode intermediate article 272 that is further reduced indiameter, formed with the flat surface F1 and a portion of the innerlayer 27A (the side to which the second noble metal tip 32 is weldedlater) is deformed into a recessed shape. Other processes are the sameas the above-described first embodiment.

In this embodiment, as shown in FIG. 8, the protrusion height A of thesecond noble metal tip 32 from the joining surface, i.e., the flatsurface F1 to the front end of the second noble metal tip 32 is set to0.4 mm or more. Further, the depth E of the melted portion 42 from theflat surface (joining surface) F1 and toward the inner layer 27A alongthe axial CL1 direction is set to 0.1 mm or more, and the minimumdistance F between the melted portion 42 and the inner layer 27A is setto 0.1 mm or more. Further, the minimum distance T between the flatsurface (joining surface) F1 and the inner layer 27A is set to 0.4 mm orless. Further, the outer diameter W of the second noble metal tip 32 andthe lateral width C of the inner layer 27A satisfy W≦C.

According to the second embodiment structure as described above, thesecond noble metal tip 32 side shape of the inner layer 27A is recessed.For this reason, as compared with the first embodiment, the meltedportion 42 can be formed so as to be further deep. Accordingly, it canbe attained to further improve the joining strength of the second noblemetal tip 32 while inhibiting lowering of the oxidation resistance.

(Confirmation of Effect)

Herein, to confirm the above-described effect, various samples wereprepared by varying the sectional area of the inner layer 27A, thesectional shape of the inner layer 27A, etc. and it was attempted toconduct various evaluations. The result of examination is described inthe following.

First, samples were such spark plug samples (samples 1 to 9) with thescrew diameter of M12, the protrusion height from the combustion chamberto the front end surface of the first noble metal tip 31 of 3.5 mm, andthe spark discharge gap of 1.05 mm, and joined with an Ir-5Pt alloy withthe diameter of 0.6 mm and the height of 0.8 mm as the first noble metaltip 31 and with a Pt-20Rh alloy with the diameter W of 0.7 mm and A of0.8 mm as the second noble metal tip 32, the samples being varied in thesectional area of the inner layer 27A, the sectional shape, etc.variously, and with the samples being installed on a 3-cylinder in-lineengine of 660 cc displacement, the engine was operated for 300 hours intotal under the test condition of 4000 rpm, full throttle, ignitiontiming of 5 BTDC and A/F (air-to-fuel ratio) of 10.7 (however, sampleswere rotated every 50 hours (cylinders were also rotated)). Then, theconsumption volume γ and the oxidation scale ratio δ of the spark plugsamples after the test were measured. In the meantime, the consumptionvolume γ indicates the reduced amount of volume of the second noblemetal tip 32 after the test from the initial volume. More specifically,the volume of the second noble metal tip 32 was measured using a CTscanner before the test, and the volume of the second noble metal tip 32was similarly measured after the test. By subtracting the volume beforethe test from the volume after the test, the consumed volume wascalculated. Further, the oxidation scale δ relates to the spark plugsample after the operation under the above-described test condition andis calculated by measuring, in a cross section of the ground electrode27 as viewed from the front end surface side of the ground electrode 27along the axis CL1, the depth (SSL+SSR) of the oxidation scale along thedirection crossing at right angles the axis CL1, which oxidation scaleis formed at the interface between the melted portion 42 and the secondnoble metal tip 32, relative to the depth (BSL+BSR) along the directioncrossing at right angles the axis CL1 on the interface between themelted portion 42 and the second noble metal tip 32, as shown in FIG.11.

The result of evaluation is shown in table 1 and table 2. However, inthe tables, while “A”, “E”, “F” have already been explained, “B”indicates the lateral width of the ground electrode 27 in the directioncrossing the axis CL1 at right angles, “C” indicates the lateral widthof the inner layer 27A in the direction crossing the axis CL1 at rightangles, and “D” indicates the distance between the point of the innerlayer 27A, which is remotest from the center electrode 5, and the pointof the outer layer 27B, which is remotest from the center electrode 5.Further, the samples 1 to 6 in the tables the second noble metal tip 32side cross sectional shape of the inner layer 27A is flat or recessed,and in contrast the samples 7 to 9 are comparative examples and theinner layer 27A has a circular cross sectional shape. More specifically,as to the samples 1 to 6, the flat surface F1 is swaged so that thecross section of the second noble metal tip 32 side shape is flat orrecessed, and as to the samples 7 to 9, the outer layer 28B which isinitially cylindrical is formed with the flat surface F1 by removal(cutting), the cross section of the inner layer 27A being not shaped soas to be flat or recessed but circular. Further, the samples, except forthe samples 1 and 7, satisfy the relation of W≦C.

TABLE 1 Flat Surface Sectional Area α Inner Layer Consumed OxidationScale Sample Formation A B C of Inner Layer Ratio D E F volume γ Ratio δNo. By Swaging (mm) (mm) (mm) C/B (mm²) β (%) (mm) (mm) (mm) (mm³) (%) 1Yes 0.8 1.3 0.6 0.46 0.251 20.23 0.35 0.2 0.1 0.0498 26 2 Yes 0.8 1.30.72 0.55 0.361 29.09 0.29 0.2 0.06 0.0344 19 3 Yes 0.8 1.3 0.9 0.690.565 45.53 0.20 0.2 0 0.0246 95 4 Yes 0.8 1.3 0.9 0.69 0.565 45.53 0.200.15 0.05 0.0276 20 5 Yes 0.8 1.3 0.9 0.69 0.565 45.53 0.20 0.1 0.10.0286 45 6 Yes 0.8 1.3 0.9 0.69 0.565 45.53 0.20 0.05 0.15 0.0253 70 7Yes 0.8 1.3 0.6 0.46 0.283 22.78 0.35 0.2 0 0.0438 92 8 Yes 0.8 1.3 0.720.55 0.407 32.81 0.29 0.2 0 0.0299 95 9 Yes 0.8 1.3 0.9 0.69 0.636 51.260.20 0.2 0 0.0234 96

TABLE 2 Hardness (Hv) Center Electrode Back Surface Sample No. Side Side1 223 195 2 235 202 3 245 210

As shown in the above-described Table 1, it is seen that there is atendency that with increase of the sectional area α of the inner layer27A, the heat drawing becomes better and the consumption volume γdecreases. However, it is revealed that if F is zero, that is, in caseof the sample 3 in which the melted portion 42 is in contact with theinner layer 27A, the oxidation scale ratio δ is remarkably large, thuspossibly causing a bad effect on the joining strength.

Further, in comparison between the sample 1 (embodiment) and the sample7 (comparative example) and between and between the sample 2(embodiment) and the sample 8 (comparative example) whose inner layersectional areas α are close to each other, if it was tried to secure “2mm” for the depth E of the melted portion 42 from the flat surface(joining surface) F1 toward the inner layer 27A along the axial CL1direction, F became zero in case of the samples 7 and 8. Accordingly, itcan be said that by forming the second noble metal tip 32 side crosssectional shape of the inner layer 27A into a flat shape or recessedshape, a larger depth E of the melted portion 42 in the axial CL1direction can be secured.

On the other hand, regarding the sample 6 (E=0.05 mm) in which the depthE of the melted portion 42 from the flat surface (joining surface) F1toward the inner layer 27A along the axial CL1 direction, was notsufficiently obtained, the oxidation scale ratio 6 became relativelylarge, i.e., 70%. In other words, in case of obtaining a larger joiningstrength, it can be said desired that the depth E of the melted portion42 from the flat surface (joining surface) F1 toward the inner layer 27Aalong the axial CL1 direction is “0.1 mm” or more.

In the meantime, as shown in Table 2, it was revealed that in thesamples 1 to 3 of this embodiment (all of them being formed with theflat surface F1 of the ground electrode 27 by swaging), the centerelectrode 5 side portion of the outer layer 27B had a larger hardnessthan that of the back surface side portion on the side opposite to thecenter electrode 5. It is considered that this is because the centerelectrode 5 side portion is larger in the rate of work, for being formedwith the flat surface F1 by swaging, thus causing the internaldistortion and increasing the hardness.

In the meantime, the content of description of the above-describedembodiment is not limitative but the following embodiments 3 to 5 can beemployed.

Third Embodiment

Then, the third embodiment will be described with reference to FIG. 12.However, in the third embodiment, the same reference characters are usedfor the same or like parts as the first embodiment while their duplicatedescription being omitted, and the different point from the firstembodiment will be mainly described.

In the first embodiment, in the cross section of the ground electrode 27as viewed from the front end surface side of the ground electrode 27along the axis CL1, only the second noble metal tip 32 side is providedwith the flat surface F1. In contrast to this, in this embodiment, theback surface on the side opposite to the flat surface F1 is alsoprovided with a flat surface F2. Further, this embodiment features thatthe inner layer 27A has a pair of flat surfaces corresponding to notonly the flat surface F1 but also the flat surface F2.

In the above-described cross section, the portions of the groundelectrode 27 except for the flat surfaces F1 and F2 are formed into apair of outwardly curved surfaces, and correspondingly to this, theinner layer 27A is also provided with a pair of outwardly curvedsurfaces. Such curved surfaces of the ground electrode 27 are providedfor promoting turning around of the mixture gas into the spark dischargegap. In the meantime, since the front end of the second noble metal tip32 protrudes more toward the first metal tip side than the circumferenceof an imaginary circle 27C that is formed by extending the pair ofcurved surfaces, the discharge voltage can be reduced.

The protrusion height A from the joining surface of the second noblemetal tip 32, i.e., the flat surface F1 to the front end of the secondnoble metal tip 32 is set to 0.4 mm or more. Further, the depth E of theabove-described melted portion 42 from the flat surface (joiningsurface) F1 toward the inner layer 27A along the axial CL1 direction isset to 0.1 mm or more, and the minimum distance F between the meltedportion 42 and the inner layer 27A is set to 0.1 mm or more. Further,the minimum distance T between the flat surface (joining surface) F1 andthe inner layer 27A is set to 0.4 mm or less. Further, the outerdiameter W of the second noble metal tip 32 and the lateral width C ofthe inner layer 27A satisfies W≦C.

Fourth Embodiment

Then, the fourth embodiment will be described with reference to FIG. 13.However, in the fourth embodiment, the same reference characters areused for the same or like parts as the first embodiment while theirduplicate description being omitted, and the different point from thefirst embodiment will be mainly described.

In the first embodiment, the second noble metal tip 32 is joineddirectly to the ground electrode 27 by laser welding. Accordingly, themelted portion 42 is formed so as to extend toward the inside of theground electrode 27. In contrast to this, in this embodiment, as shownin FIG. 13, an intermediate member 43 is provided between the secondnoble metal tip 32 and the ground electrode 27. The intermediate member43 is made of nickel alloy similarly to the outer layer 27B of theground electrode 27. A base end portion of the intermediate member 43 isjoined to the flat surface F1 by resistance welding. On the other hand,the base end portion of the intermediate member 43 is joined to thesecond noble metal tip 32 by laser welding. The melted portion 42 isformed at the interface between the second noble metal tip 32 and theintermediate member 43 and at a distance from the flat surface F1.Accordingly, even if the minimum distance T between the flat surface F1and the melted portion 42 is made smaller, it is possible to prevent themelted portion 42 from reaching the inner layer 27A. Further, since ascompared with the first embodiment, the volume of the second noble metaltip 32 can be made smaller, the amount of expensive noble metal used canbe reduced.

The protrusion height A from the joining surface of the intermediatemember 43, i.e. the flat surface F1 to the front end of the second noblemetal tip 32 is set to 0.4 mm or more. Since the front end of the noblemetal tip 32 protrudes more toward the first noble metal tip 31 sidethan the circumference of the imaginary circle 27C that is formed byextending the circular arc shape of the back surface of the groundelectrode 27, the discharge voltage can be reduced. Further, the minimumdistance T between the flat surface (joining surface) F1 and the innerlayer 27A is set to 0.4 mm or less. Further, the outer diameter W of thesecond noble metal 32 and the lateral width C of the inner layer 27Asatisfy W≦C.

Fifth Embodiment

Then, the fifth embodiment will be described with reference to FIG. 14.However, in the fifth embodiment, the same reference characters are usedfor the same or like parts as the fourth embodiment while theirduplicate description being omitted, and the different point from thefirst embodiment will be mainly described.

In the fourth embodiment, the flat surface F1 is provided only on theintermediate member 43 side. In contrast to this, in this embodiment, asshown in FIG. 14, also the back surface on the side opposite to the flatsurface F1 is provided with a flat surface F2. Further, this embodimentfeatures that the inner layer 27A has a pair of flat surfacescorresponding to not only the flat surface F1 but also the flat surfaceF2.

In the above-described cross section, the portions of the groundelectrode 27 except for the flat surfaces F1 and F2 are formed into apair of outwardly curved surfaces, and correspondingly to this, theinner layer 27A is also provided with a pair of outwardly curvedsurfaces. Such curved surfaces of the ground electrode 27 are providedfor promoting turning around of the mixture gas into the spark dischargegap. In the meantime, since the front end of the second noble metal tip32 protrudes more toward the first noble metal tip 31 side than thecircumference of an imaginary circle 27C that is formed by extending thepair of curved surfaces, the discharge voltage can be reduced.

The protrusion height A from the joining surface of the intermediatemember 43, i.e., the flat surface F1 to the front end of the secondnoble metal tip 32 is set to 0.4 mm or more. Further, the minimumdistance T between the flat surface (joining surface) F1 and the innerlayer 27A is set to 0.4 mm or less. Further, the outer diameter W of thesecond noble metal tip 32 and the lateral width C of the inner layer 27Asatisfy W≦C.

(Confirmation of Effect)

Further, in order to examine the effect of the relation between theouter diameter W of the second noble metal tip 32 and the lateral widthC, various samples were prepared by holding the outer diameter W of thesecond noble metal tip 32, the total sectional area of the groundelectrode 27 and the sectional area of the inner layer 27A constant andvarying the lateral width B of the ground electrode 27 and the lateralwidth C of the inner layer 27A and it was attempted to conduct variousevaluations. The result of examination is described in the following.

First, samples were such spark plug samples (samples 10 to 13) with thescrew diameter of M12, the protrusion height from the combustion chamberto the front end surface of the first noble metal tip 31 of 3.5 mm, andthe spark discharge gap of 1.05 mm, and joined with Ir-5Pt alloy withthe diameter of 0.6 mm and the height of 0.8 mm as the first noble metaltip 31 and with Pt-20Rh alloy with the diameter W of 0.7 mm and A of 0.8mm as the second noble metal tip 32, the samples being varied in thelateral width B of the inner layer 27A and the lateral width B of theinner layer 27A variously, and with the samples being installed on a3-cylinder in-line engine of 660 cc displacement, the engine wasoperated for 300 hours in total under the test condition of 4000 rpm,full throttle, ignition timing of 5° BTDC and A/F (air-to-fuel ratio) of10.7 (however, samples were rotated every 50 hours (cylinders were alsorotated)). In the meantime, the protrusion height H of the intermediatemember 43 from the joining surface (flat surface F1) is 0.35 mm, and thelength of the second noble metal tip 32 is 0.45 mm. Then, theconsumption volume γ and the oxidation scale ratio δ of the spark plugsamples after the test were measured. In the meantime, the consumptionvolume γ indicates the reduced amount of volume of the second noblemetal tip 32 after the test from the initial volume. More specifically,the volume of the second noble metal tip 32 was measured using a CTscanner before the test, and the volume of the second noble metal tip 32was similarly measured after the test. By subtracting the volume beforethe test from the volume after the test, the consumed volume wascalculated.

Further, the sample 10 in Table has only the flat surface F1 as shown inFIG. 13, and on the contrary the samples 11 to 13 have the flat surfaceF1 and the flat surface F2 as shown in FIG. 14. Except for the sample10, the samples satisfy the relation of W≦C.

TABLE 3 Total Sectional Flat Surface Area of Ground Sectional Areaconsumption Sample Formed By A B C Electrode of inner layer T Volume γNo. Swaging (mm) (mm) (mm) (mm²) (mm²) (mm) (mm³) 10 F1 0.8 1.3 0.61.124 0.251 0.3 0.0526 11 F1, F2 0.8 1.4 0.7 1.242 0.251 0.3 0.0481 12F1, F2 0.8 1.45 0.8 1.243 0.252 0.305 0.0473 13 F1, F2 0.8 1.5 0.9 1.2450.259 0.308 0.047

As shown in the above-described Table 1, it will be seen that even ifthe total sectional area of the ground electrode 27 and the sectionalarea a of the inner layer 27A are substantially the same, the heatdrawing becomes better with increase of the lateral width C of the innerlayer 27A.

In the meantime, the content of the description of the above-describedembodiments are not for the purpose of limitation but variousmodifications may be made thereto as described in the following.

(a) While in each of the above-described embodiments is utilized theground electrode 27 having substantially the same cross sectional shapethroughout the extent in the longitudinal direction, a ground electrode27 having, as shown in FIG. 15, a base portion 71 joined to the frontend surface of the metallic shell 3 and having a substantiallyrectangular cross sectional shape of a constant width, a small diameterportion 72 having a circular cross section (however, provided with aflat surface) and positioned more on the front end side than the baseportion 71, and a tapered portion 73 gradually varying in the crosssectional shape along the longitudinal direction (however, in thefigure, a center electrode, etc. are omitted) may be employed. In thiscase, increase in the joining area between the ground electrode 27 andthe metallic shell 3, in its turn, increase in the joining strength canbe attained.

In short, there is not any particular limitation on the shape of theground electrode 27, provided that it has more on the front end sidethan the spark discharge gap 33 and at the back surface on the sideopposite to the center electrode 5 side and/or at the side surface anoutwardly curved surface.

(b) Further, while in each of the above-described embodiments the groundelectrode 27 is shaped so as to have the flat surface F1 throughout theextend in the longitudinal direction, the portion more on the front endside than the bent portion of the ground electrode 27 may be swaged soas to be formed into a substantially flat surface shape. Further, atleast only the portion to which the second noble metal tip 32 is joinedmay have the flat surface F1.

(c) It will suffice that the portion to which the second noble metal tip32 is joined is substantially flat surface-shaped, and it is not neededto be of a flat surface in the strict sense of the word. Accordingly, itdoes not matter that the portion has some undulations.

(d) It will suffice to use, as a core member that constitutes the innerlayer 27A, one that has a recessed shape from the beginning, though notreferred to in each of the above-described embodiments. Further, it willsuffice to dispose the inner layer 27A at an eccentric position withrespect to the outer layer 27B.

(e) In the above-described embodiments, there is shown a cross sectionin which the melted portion 42 on one side is not connected with that onthe other side, they may be connected with each other.

1. A spark plug comprising: a rod-shaped center electrode; a first noblemetal tip joined to a front end of said center electrode; asubstantially cylindrical insulator provided at an outer circumferenceof said center electrode; a tubular metallic shell provided at an outercircumference of said insulator; a ground electrode having a base endportion joined to a front end surface of said metallic shell, and afront end portion facing a front end portion of said center electrode,said ground electrode including an outer layer made of nickel alloy andan inner layer made of a material having a better thermal conductivitythan the outer layer; and a second noble metal tip joined to said frontend portion of said ground electrode by way of a melted portion formedby one of laser welding and beam welding and forming a spark dischargegap between said second noble metal tip and said first noble metal tip,wherein in a cross section of said ground electrode as viewed from thefront end surface side of said ground electrode along the axis of saidsecond noble metal tip; the protrusion height A of said second noblemetal tip from said joining surface to said front end of said secondnoble metal tip is 0.4 mm or more; said ground electrode includes asubstantially flat joining surface to which said second noble metal tipis joined and an outwardly curved surface; said inner layer has at saidjoining surface side one of a substantially flat surface and recessedsurface; and the minimum distance F between the said melted portion andsaid inner layer is 0.1 mm or more.
 2. A spark plug according to claim1, wherein in said cross section, the depth E of said melted portionfrom said joining surface toward said inner layer along said axialdirection is 0.1 mm or more.
 3. A spark plug comprising: a rod-shapedcenter electrode; a first noble metal tip joined to a front end of saidcenter electrode; a substantially cylindrical insulator provided at anouter circumference of said center electrode; a tubular metallic shellprovided at an outer circumference of said insulator; a ground electrodehaving a base end portion joined to a front end surface of said metallicshell, and a front end portion facing a front end portion of said centerelectrode, said ground electrode including an outer layer made of nickelalloy and an inner layer made of a material having a better thermalconductivity than the outer layer; a second noble metal tip joined tothe front end portion of said ground electrode by way of a meltedportion formed by one of laser welding and beam welding and forming aspark discharge gap between the second noble metal tip and the firstnoble metal tip, wherein in a cross section of said ground electrode asviewed from the front end surface side of the ground electrode along theaxis of said second noble metal tip, the protrusion height A of saidsecond noble metal tip from said joining surface to the front end ofsaid second noble metal tip is 0.4 mm or more; said ground electrodeincludes a substantially flat joining surface to which said second noblemetal tip is joined and an outwardly curved surface; said inner layerhas at a side of said joining surface one of a substantially flatsurface and recessed surface; and said melted portion is disposed at adistant from said joining surface.
 4. A spark plug according to claim 3,wherein in said cross section, the shortest distance between saidjoining surface and said inner layer is smaller than the protrusionheight H from said joining surface of said intermediate member.
 5. Aspark plug according to claim 1, wherein in said cross section, theshortest distance T between said joining surface and said inner layer is0.4 mm or less.
 6. A spark plug according to claim 1, wherein assumingthat in said cross section, W denotes the width of said front endsurface of said second noble metal tip and C denotes the width of saidinner layer, in the direction parallel to said joining surface, it issatisfied W≦C.
 7. A spark plug according to claim 1, wherein in saidcross section, a portion of said outer layer, which is on a side of saidjoining surface has a larger hardness than a portion on a back surfaceside which is a side opposite to said joining surface.
 8. A spark plugaccording to claim 1, wherein said curved surface has a circular arcshape, and said front end of said second noble metal tip protrudes moretoward a side of said first noble metal tip than an imaginary circlethat is formed by extending said circular arc shape of said curvedsurface.